Darpa have created an invisibility cloak for sound. This deceivingly simple looking tech could hide objects from sonar although its creation was anything but simple and required some complex math. Engineers from Duke University in Durham, North Carolina, claim that unlike other efforts, the acoustic cloaking device works in all three dimensions,no matter which direction a sound is coming from at this structure it will not give a reliable sound bounce back.

This device looks like a layered pyramidal structure and is constructed using plastic plates with a repeating pattern of holes that are stacked on top of one another. Its design means that sound waves can be manipulated by plastic and air.
The cloak alters the sound waves’ trajectory to match what they would look like had they had reflected off a flat surface.
This is due to the sound wavesnot penetrating the pyramid, but they are travelling a shorter distance, which in turn affects the wave's speed. The device then reroutes these slower sound waves to create the impression that both the cloak and anything beneath it are not there.

below is an image of the device .


Research scientist Bogdan Popa with a 3D acoustic cloaking device constructed with components created in a 3-D printer (Photo: Duke Photography)

Bogdan Popa, a graduate student in electrical and computer engineering, shows off the 3D acoustic cloak he helped design and build as a member of Steven Cummer’s laboratory. If you ever wanted to dupe an enemy sonar into thinking their sonar is hitting nothing this new cloaking device is perfect for such need, this is a mathematical marvel of engineering.

Professor of electrical and computer engineering at Duke, Steven Cummer, explains on Duke’s website that 'the particular trick we’re performing is hiding an object from sound waves.” “By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path,'

Accomplishing this nifty trick would be difficult without harnessing the power of metamaterials, which result from combining existing materials in ways that force them to alter their properties and behave unnaturally. The acoustic cloak is essentially just plastic and air cleverly combined, according to Cummer. Once the sound hits the structure, the result of painstaking mathematical calculations by engineers comes into play. It goes as far as fooling the sound as to how far it travels, making it compensate for its surroundings. The deceptively simple design is anything but, the professor explains.

“I promise you that it’s a lot more difficult and interesting than it looks. We put a lot of energy into calculating how sound waves would interact with it. We didn’t come up with this overnight,” Cummer said.

The project was clearly a success for Duke, who tested it on a small cloaked sphere and bounced sounds off of it with a microphone, then doing the same without the device. Prospects for development are, understandably, endless the cloak’s creators believe. The cloak could in future be used to fool enemy sonar, as well as for things like architectural acoustics, to compensate for the conflict between sound and the structural requirements of a building.

But this is not the first time the military has thought of something like this. Of course, the ability to cheaply 3D-print it is a huge bonus, but the concept of using metamaterial, as well as the principle of making sound pass around an object go back some time.

America’s Defense Advanced Research Projects Agency (DARPA) has been at it for ages, announcing in 2007 that they’d like to be able to make a whole military vehicle inaudible. In their 2008 defense budget they write that the effort will use directed ultrasound technology to enable the capability to significantly reduce sound emissions from large scale tactical military hardware. Theory predicts that nonlinear effects of high-power acoustic radiation on the atmosphere can cause acoustic energy to dissipate, rather than radiate.